I like to use Module::Find. It's a little slow, but if your process is long-running (like a daemon or mod_perl app) it's sufficient. It lets you easily load all modules under a given namespace. Then you can add plugins just by dropping a new module in the right spot.

You still have to do some work to use the plugins, of course. Either your client code needs to keep track of the modules loaded, or needs to call some initialization method in them, or the modules need to register themselves with the client code when they're loaded.